The American College of Sports
Medicine has produced guidelines regarding how a
race should consider reacting to various weather
conditions. The ACSM has purposely avoided creating a
strict cancellation standard, stating that it does not
want to handcuff a race director; rather it offers
recommendations about the course of action that should
be taken in various weather conditions. The temperatures
in the ACSM guidelines are all WBGTs (wet bulb globe
temperatures). The WBGT is an index that takes into
account heat (dry bulb), humidity (wet bulb), and the
ambient temperature (black bulb, which factors in
additional variables such as reflected heat and wind). A
WBGT thermometer can be purchased from a medical supply
house for about $100. The ACSM guidelines are as
follows:

WBGT >82° F (>28° C) Black Flag:
Cancel or recommend voluntary withdrawal. (Races that
are always held in these conditions should acknowledge
the extreme heat risk to the poorly acclimated and
non-resident competitors in the pre-race literature and
the pre-race announcements)